752 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



N° 3 still remained unaltered at some small distance from the spectrum. This 

 quantity was full as much as I could expect, considering the heat that must have 

 been intercepted by 3 prisms. Thus then it appears, that the different refrangi- 

 bility of heat, as well as that of light, admits of prismatic correction. And we 

 may add, that this experiment also tends to the establishment of the contents of 

 the preceding one ; for the refrangibility of heat rays could not be thus corrected, 

 were the sines of refraction not in a constant ratio to those of incidence. 



Exper. 23. In Burning-glasses, the Focus of the Rays of Heat is Different 

 from the Focus of the Rays of Light. — I placed the burning lens, with its aperture 

 reduced to 3 inches, in order to lessen the aberration arising from the spherical 

 figure, in the united rays of the sun ; and being now apprised of the different 

 refrangibility of the rays of heat, and knowing also that the least refrangible of 

 them are the most efficacious, I examined the focus of light, by throwing hair- 

 powder, with a puff, into the air. This pointed out the mean focus of the illu- 

 minating rays, situated in that part of the pencil which opticians have shown to be 

 the smallest space into which they can be collected. That this may be called the 

 focus of light, our experiments, which have proved the maximum of illumination 

 to be situated between the yellow and green, and therefore among the mean refran- 

 gible rays of light, have fully established. The mean focus being thus pointed 

 out by the reflection of light on the floating particles of powder, I held a stick 

 of sealing-wax l s .6 or 4 beats of my chronometer, in the contracted pencil, half 

 an inch nearer to the lens than the focus. In this time, no impression was made 

 on the wax. I applied it now half an inch farther from the lens than that focus ; 

 and, in tV of a second, or 2 beats of the same chronometer, it was considerably 

 scorched. Exposing the sealing-wax also to the focus of light, the effect was 

 equally strong in the same time ; from which we may safely conclude, notwith- 

 standing the little accuracy that can be expected, for want of a more proper appa- 

 ratus, from so coarse an experiment, that the focus of heat, in this case, was 

 certainly farther removed from the lens than the focus of light, and probably not 

 less than ■{- of an inch ; the heat, at half an inch beyond the focus of light being 

 still equal to that in the focus. 



Art. 5. Transmission of Heat-making Rays. — We enter now on the sub- 

 ject of the transmission of heat through diaphanous bodies. Our experiments 

 have hitherto been conducted by the prism, the lens, and the mirror ; these may 

 indeed be considered as our principal tools, and, as such, will stand foremost in all 

 our operations ; but the scantiness of this stock cannot allow us to bring our work 

 to perfection. Nor is it merely the want of tools, but rather the natural imper- 

 fection of those we have, that hinders onr rapid progress. The prism which we 

 use for separating the combined rays of the sun, refracts, reflects, transmits, and scat- 

 ters them at the same time ; and the laws by which it acts, in every one of these 

 operations, ought to be investigated. Even the cause of the most obvious 

 of its effects, the separation of the colours of light, is not well understood ; for, 



